The cost of sending a single person seeking asylum to Rwanda could be nearly £170,000, according to government analysis, which has immediately reignited bitter rows over the controversial scheme. A long-awaited “impact assessment” of the illegal migration bill has conceded that ministers do not know the overall costs of implementing plans to detain and deport anyone who arrives in the UK by irregular means. The disclosure comes at the start of a pivotal week for the government’s flagship policy, which is meant to “stop the boats”, one of Rishi Sunak’s five key promises. Peers have already threatened to derail the bill when it returns to the upper chamber on Wednesday. On Thursday, the court of appeal is expected to rule on whether it is legal to deport people seeking asylum, including women and children, to Rwanda. Enver Solomon, the head of the Refugee Council, said the assessment failed to evaluate the true costs and consequences of the government’s proposed legislation. “If enacted in its current form, the bill would leave tens of thousands of refugees unable to access the protection they are entitled to under international law. It would cause hardship, cost billions of pounds, and do nothing to alleviate the current crisis and pressures within the asylum system,” he said. Suella Braverman, the home secretary, said the assessment examining the costs of the illegal migration bill showed the government would save at least £106,000 for every person deterred from entering the UK by irregular means. “Our impact assessment shows that doing nothing is not an option. “I urge MPs and peers to back the bill to stop the boats, so we can crack down on people smuggling gangs while bringing our asylum system back into balance,” she said. But the assessment also claimed that locating an individual to the central African country or another third country is estimated to cost £169,000, based on a previous government scheme. It said that “an estimated unit cost of £169,000 is found for relocating an individual … This cost will only be incurred for people who arrive in the UK illegally.” A Home Office source said the figure is based on a “theoretical exercise on costs under the bill”. The assessment said it is “not possible to estimate with precision the level of deterrence” the bill will have. It noted academic consensus is that there is “little to no evidence” policy changes deter people leaving their home countries and seeking refuge. Instead, shared language, culture and family ties were accepted to be “strong factors” influencing choice of final destination. The Home Office has refused to publish the payments agreed with the Rwandan government, citing “commercial sensitivities”, on top of a £140m payment handed over as part of a deal signed under Boris Johnson’s government. In passages that undermine the government’s plan to detain and deport everyone who arrives by irregular means, the assessment said constraints on capacity meant the illegal migration bill might only be applied to a proportion of arrivals. “This could lead to additional costs associated with bailing individuals and providing non-detained accommodation, a reduced deterrent effect observed and further process issues such as migrants absconding while not detained,” the report said. Braverman’s plan to house people seeking asylum on barges was called “unworkable” on Monday as she missed her own target for the first vessel to be in place. The Bibby Stockholm accommodation vessel, which would house about 500 people, was not yet in Portland Port, Dorset, despite the home secretary’s promise it would be in the dock a week ago. The release of the document came after peers threatened to delay the bill until after the government had published facts and figures showing the financial impact. The government awaits a court of appeal judgment on the legality of its plans to deport people seeking asylum including women and children to Rwanda. Organisations, including the United Nations high commissioner for refugees, have warned the proposals are in breach of international law and set a dangerous precedent for other countries in their treatment of people seeking refuge. Yvette Cooper, the shadow home secretary, said the assessment was a “complete joke” and that the illegal migration bill could cost taxpayers billions more. “By its own admission, this failing Conservative government is totally clueless on how much this bill will cost or what the impact of any of its policies will be. “The few figures the Home Office has produced show how chaotic and unworkable their plans are. It suggests that if Rishi Sunak were actually able to deliver on his promise to remove every asylum seeker who arrives in the UK it would cost billions of pounds more even than the Tories’ broken asylum system today,” Cooper said. Shami Chakrabarti, the human rights barrister who has led criticisms of the bill in the Lords, compared the assessment to a press release. “Is this the much awaited document, so carefully prepared that it could not be published at any stage during such a contentious bill’s passage through the House of Commons? “The so-called impact assessment is about the length of a Home Office press release and similar in style and content. It’s all about deterrence but the financial and human costs are ‘untested’, the delivery plan still ‘being developed’ and international law is completely ignored,” she said.